The concept of the ischemic penumbra is now more than 20 years old. Following occlusion of a brain artery some of the brain tissues supplied by the vessel perish due to hypoxia/anoxia but some tissues have a capability to recover with appropriate treatment. Astrup first defined ischemic penumbra in 1981 as perfused brain tissue at a level within the thresholds of functional impairment and morphological integrity, which has the capacity to recover if perfusion is improved. This happens due to the inability of cells to produce ATP (energy) leading to cell dysfunction and then cell death. Therefore any technique demonstrative of active metabolism within the affected tissues would also be able to detect the penumbra.